The characteristics of THM formation in a water distribution system is inspected by the analysis of measured data such as chlorine residual, DOC, UV_(254), temperature and pH. The result shows that THM concentration increases by 30% or less in the distribution system as the distance from water treatment plant increases. THM formation also increases with the increase of temperature, DOC, and UV_(254). On the contrary to the theoretically obtained result, in which THM formation increases as pH increases, the THM formation in the distribution system decreases as pH increases. This is caused by the complicated interactions among the factors of water quality. It is shown that UV_(254), is more effective than TOC or DOC as a measure of THM precursors. In addition, the effect of water quality on other disinfection by-products should be considered when THM formation is intended to be reduced by controlling water quality.